EFFECT OF SLAG PREPARED WITH DIFFERENT COOLING METHODS ON CLEANLINESS OF BEARING STEEL GCr15
Dong-ping Zhan1, Yang-peng Zhang1, Lei Tang1, Kun Fan1, Zhou-hua Jiang1, Hui-shu Zhang2
1School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China;
2School of Metallurgical Engineering, Liaoning Institute of Science and Technology, Benxi,
117004
Keywords: Bearing Steel, Different Cooling Methods Slag, Cleanliness Abstract
Bearing steel required high cleanliness. During LF refining, the type of slag has obvious effect on its cleanliness. Two kinds of refining slags have been prepared with different cooling methods after pre-melting. Another slag was used by only mixing the raw materials .Then six heats steelmaking experiments were carried out in the MoSi2 electric resistance furnace to study their effect on the cleanliness of bearing steel GCr15. The results show that: The water-cooled slag is full of glass phase or microcrystal phase inside, and it has the lowest melting point, the shortest melting time, the fastest desulfurization velocity and the smallest average size of inclusions.
Those effects of the air-cooled slag are better than the mixed-slag's. Under the same process, the inclusion numbers and area in the steel refined by the slag with CaO/Al2O3=1.65 are smaller than those of the slag with CaO/Al2O3=0.94.
Introduction
With the development of modern industrial, excellent abrasive resistance, higher fatigue resistance of bearing steel are required. To meet the requirements, on the one hand, the content of inclusions and mass fraction of harmful elements like oxygen and sulfur must be decreased very low, on the other hand, the composition and morphology of inclusions should be controlled to ensure the good ability of plastic deformation [1-10]. In order to achieve the purpose, the refining slag could melt in a short time and achieve the purpose of desulfurization and adsorbing inclusions [11,12].
Metallurgical workers have done a lot of research works on the refining slag system. Wang Deyong [11] found that the refining slag with active Al2O3 and aluminum ash can significantly improve the desulfurization rate and reduce the average time of LF process. A composite slag with CaO, Al2O3 and CaF2 has a low melting point and good desulfurization rate, what’s more, the ability of adsorbing inclusions is also very good[12,13]. The DaiDo steel did some experiments[14,15], and the results showed that the mass fraction ratio of slag and metal between 1% and 3%, the basicity of slag is in 4.0 - 4.5, and the total weight of FeO and MnO in the slag is less than 1% is the suitable desulfurization condition. The study on the optimizing of refining slag composition is very mature, but the purpose of better adsorbing ability and melting speed has not been satisfied, this needs us in other ways to improve the effect of the refining slag.
At present the research about preparation methods are not much, therefore, to study this topic in this aspect, different refining with different cooling method were prepared. The metallurgical properties (melting speed and desulfurization) was studied. At the same time, different
Advances in Molten Slags, Fluxes, and Salts: Proceedings of The 10th International Conference on Molten Slags, Fluxes and Salts (MOLTEN16) Edited by: Ramana G. Reddy, Pinakin Chaubal, P. Chris Pistorius, and Uday Pal TMS (The Minerals, Metals & Materials Society), 2016
composition refining slag were prepared to study the influence on the adsorbing ability and properties of inclusions, that caused by the basicity and ratio of calcium and aluminum of the refining slag. The results in this paper shows a new way to control the cleanliness and inclusions in the GCr15 steel, even all the bearing steel, and has the obvious significance.
Experimental
The slag system used for our experimental is CaO-SiO2-Al2O3. In the fact, the refractory material could reveal and to meet the flowing property, some MgO and CaF2 were added in the system.
But because the mass fraction of MgO and CaF2 is very little and stable, so the slag can be treated as CaO-SiO2-Al2O3 slag system during our study. Fig. 1 shows the CaO-SiO2-Al2O3
phase diagram.
Fig. 1 Ternary phase diagram of CaO-SiO2-Al2O3
As shown in Fig. 1, the composition range of elliptic area is 40%~60%CaO, 0%~10%SiO2, 30%~60%Al2O3, which is the low melting point area. In order to control the inclusions in bearing steel in 12CaO·7Al2O3 and 3CaO·Al2O3’s low melting point area, and desulfurization conditions of high basicity and low oxygen potential. With CaO/Al2O3 ratio of variable, we design two experiences that the ratio of CaO/Al2O3 are 0.94 and 1.65 respectively. One slag is mechanical mixed by the raw materials, CaO, SiO2, MgO, Al2O3, CaF2 and so on. The other slags are water-cooled or air-cooled after smelted in a MoSi2 eclectric-resistance furnace at 1873 K respectively.
Table 1 Scheme of tests on the refining slag of bearing steel
Number Compositions /%
Basicity CaO /Al2O3
Process CaO SiO2 Al2O3 MgO CaF2
S1 40.7 8 43.3 5 3 5.09 0.94 Water-cooled
S2 40.7 8 43.3 5 3 5.09 0.94 Air-cooled
S3 40.7 8 43.3 5 3 5.09 0.94 mechanical mixed
S4 52.3 8 31.7 5 3 6.54 1.65 Water-cooled
S5 52.3 8 31.7 5 3 6.54 1.65 Air-cooled
S6 52.3 8 31.7 5 3 6.54 1.65 mechanical mixed
The difference between water-cooled slag and air-cooled slag is the cooling method. Both water- cooled slag and air-cooled slag were melted by the MoSi2 furnace first, and the only difference between them is the cooling methods. The water-cooled slag was cooled by water and the other one was cooled by air. When the different type slags were prepared, we analysis the
crystallization ratio by XRD and determinate the melting point and melting speed.
To study the effect of different slags on the smelting process of Gr15 bearing steel, these six type slags were used for a slag-metal equilibrium experiment respectively. The raw material used for these six experiment is shown in table 2
Table 2 Composition of Bearing steel (GCr15) (mass fraction /%)
Element C Si Mn P S Cr
Content 0.95 0.25 0.33 0.010 0.004 1.50
Results and Discussion
3.1 XRD analysis
Fig. 2 shows the XRD results of the water-cooled slag and air-cooled slag. Table 3 shows the slag crystallinity analyzed by MdI Jade 6 software. Air-cooled slag presents obviously sharp diffraction peak. However, water-cooled slag has no obvious diffraction peak, and it present a big steamed bread peak in the range of 20°~50°. A large percentage of air cooled slag has been crystallized. The refining slag made by water-cooled process is mainly composed of glass phase or microcrystalline phase and the phase in the slag can be ignored.
20 40 60 80
0 200 400 600 800 1000
Intensity /a.u.
2Theta /degree
20 40 60 80
0 1000 2000 3000 4000 5000 6000
■■■■
■
■
■
■
■
■
Intensity /a.u.
2Theta /degree
■ Ca12Al14O32F2
■
20 40 60 80
0 200 400 600 800 1000
Intensity /a.u.
2Theta /degree 20 40 60 80
0 1000 2000 3000 4000 5000 6000
● ●
●
●
■ ■
■
Intensity /a.u. ■
2Theta /degree
■●
■Ca4Si2O7F2
●Ca2Al2SiO7
Fig. 2 XRD results of different slag.
(a) water cooled, CaO/Al2O3= 0.94;(b) air cooled slag with CaO/Al2O3= 0.94; (c) water cooled slag with CaO/Al2O3= 1.65; (d) air cooled slag with CaO/Al2O3= 1.65;
(a)
(c)
(b)
(d)
Table 3 Crystallinity of different refining slags Number Crystallization ratio /%
S1 0.5
S2 98.36
S4 0.2
S5 95.29
3.2 Melting Property
Melting temperature changes of different bearing steel refining slag are shown in Fig. 3.
1200 1250 1300 1350 1400 1450
75 70 60 50 40 25 10
Temperture /℃
The melting ratio /%
S1 S4 S2 S5 S3 S6
5
Fig. 3 Melting temperatures of different refining slags
It can be seen from the data that the melting points of S1, S2 and S3 slag successively increases, and the melting points of S4, S5 and S6 slag have the sam trend. Slags with the same
composition have the order of the melting point of water-cooled slag, air-cooled slag and mechanical-mixed slag. The melting point of the slag with CaO/Al2O3=0.94 is higher than that of CaO/Al2O3=1.65 when they are made by the same process. Variations of slag melting time at 1773K are shown in Fig. 4.
0 10 20 30 40 50 60 70 80 90
50% 75%
25%
Time /s
Melting degree S1 S4
S2 S5 S3 S6
5%
Fig. 4 Melting times of different refining slags
3.3 Deoxidation and Desulfuration
Steel samples of the steelmaking process are detected by oxygen and nitrogen analyzer. The variation of total oxygen content are shown in Fig. 5.
0 10 20 30 40 50 60 70 15
20 25 30 35 40 45 50
Total oxygen /10-6
Time /min
S1 S4 S2 S5 S3 S6
Fig. 5 Tendency of the mass fraction of total oxygen content
As can be seen from Fig. 5, the total oxygen content shows a slow decline and eventually reaching equilibrium value. Under the same preparation technology, the slag with
CaO/Al2O3=1.65 has faster deoxidation rate, and lower final balanced oxygen content. This is due to the less Al2O3 content in this slag, which promotes the combination of [Al] and [O].
0 10 20 30 40 50 60 70 80
0 20 40 60 80 100
Desulfurization degree /%
Time /min
S1 S4 S2 S5 S3 S6
Fig. 6 Variation of desulfuration rate with time for different slags
The variations of the desulfurization rate are shown in Fig. 6. The slag with CaO/Al2O3=1.65 has the higher desulfurization rate, and the highest one reaches 79.18%. For the same composition slag made by different processes, the order of the desulfurization rate from high to low is water- cooled slag, air-cooled slag and then mechanical mixed.
3.4 Effect of refining slag on inclusions
The size distribution of inclusion in the bearing steels is shown in Fig. 7.
0 10 20 30 40
>5.0 3.0~5.0 2.0~3.0 1.5~2.0 1.0~1.5 0.5~1.0
Distribution proportion /%
Size of inclusions /μm S1 S2 S3 S4 S5 S6
Fig. 7 Size distribution of inclusion in the bearing steels
It can be seen from Fig. 7 that the amount of small inclusions in the steel with slag blended by machine is smaller than those of water-cooled slag and air-cooled slag. On the contrary, the amount of big inclusion in the steel smelted by mechanical mixed slag is the largest, and thus the average size of the inclusions in it is the biggest.
The inclusion in the steel treated by water-cooled slag has the smallest mean size. The analysis of the steel with the same preparation process and different slag composition shows that the number of large inclusions in the steel whose CaO/Al2O3 is 1.65 is smaller than that in the steel whose CaO/Al2O3 is 0.94 and the mean size is smaller. Among the slags with same treatment, the mean size of inclusions in the steel with water-cooled slag is smaller, and the inclusions distribute more uniform, and the percentage of the area of inclusion is also minimal. Under same preparation process, the number of inclusion in steel with the refining slag whose number of CaO/Al2O3 is 1.65 is smaller when number of CaO/Al2O3 is 0.94, and the area percentage is smaller.
Through analysis, the crystalline refining slags, which is same to amorphous refining slags on composition, the inclusions has little difference in their composition. But the difference between two kinds of CaO/Al2O3 inclusions and those in the bearing steel is large. In the bearing steel used the refining slag whose number of CaO/Al2O3 is 1.65, inclusions of spinel compared with those of number of CaO/Al2O3 is 0.94 decreased, while the inclusions of Al2O3•MgO•SiO2•CaO increased. There was hardly any SiO2 and MnS. Fig. 8 is picture of some typical inclusions.
（a）Al2O3·MgO
Element Weight% AT%
O 38.992 58.175
Mg 5.313 5.218
Al 9.922 8.778
Si 17.723 15.063
S 1.867 1.390
Mn 26.183 11.376
（b）Al2O3·MgO·SiO2·TiOX·MnS Fig. 8 Typical morphology of inclusion
Conclusions
(1) Different cooling methods have significant effect on the physical and chemical properties of refining slag and crystal structures. The water-cooled slag is composed of glass phase or microcrystal phase, while the air-cooled slag crystallizes absolutely. The melting point of the water-cooled slag is the lowest and their melting speed is the highest. Among the slags with the same treatment, the refining slag whose CaO/Al2O3 is 0.94 has the higher melting point and lower melting speed.
(2) Under the same preparation process, the slag with CaO/Al2O3 is 1.65 has better the dioxidization and desulphurization effect, and has more lower oxygen content.
(3) The heat smelted by water-cooled slag with CaO/Al2O3 is 1.65 gets the best inclusions. The total number of inclusions is the lowest and the mean size is smallest. Most of the inclusions are Al2O3•MgO•SiO2•CaO.
Acknowledgements
The authors would like to appreciate National Natural Science Foundation of China [51574063]
and National High Technology Research and Development Program [2015BAF03B01] and PhD Research Startup Foundation of Liaoning Institute of Science and Technology[1406B06] and Program for Liaoning Excellent Talents in University [LJQ2015056] for providing funding for this research.
Corresponding author: Dong-ping Zhan, Tel: +86−024−83687723; E-mail: zhandp1906@163.com References
1. M.Q. Yu, “The Development Status of Bearing Steel and Its Grades Series,” Shanghai Metals, 30(3) (2008), 49-54.
2. Y.D. Li et al., “Correlations Between Very High Cycle Fatigue Properties and Inclusions of GCr15 Bearing Steel,” Acta Metallurgica Sinica, 44(8) (2008), 968-972.
3. J. Chakraborty, D. Bhattacharjee, I. Manna, “Austempering of Bearing Steel for Improved Mechanical Properties,” Scripta Materialia, 59(2) (2008), 247-250
4. K.F. Hille, F.R. Papay, N. Genma, “Slag Control Techniques for High Quality Steel” (Paper Presented at 74th Steelmaking Conference Procedings, Warrendale 1991), 419-422.
Element Weight% AT%
O 51.501 66.766
Mg 6.206 5.296
Al 14.201 10.916
Si 15.005 11.081
Ca 7.147 3.698
Mn 5.940 2.243
5. X. Yuan, M.L. Qi, “Prediction of Bearing Steel Deformation Resistance Based on RBF Neural Network,” Journal of Iron & Steel Research, 23(2) (2011), 48-52
6. Tardy P, Tolnay L, Karoly G, et al., “Bearing Steels: Cleanliness or Inclusion Modification”
(Paper Presented at The Sixth International Iron and Steel Congress Meeting. Nagoya, Japan,1990), 629-636.
7. B.H. Yoon, K.H. Heo J.S. Kim, “Improvement of Steel Cleanliness by Controlling Slag Composition,” Ironmaking and Steelmaking, 30(2) (2003), 51-59.
8. L. Liu, “Development of Process and Equipment of RH Vacuum Refinery Technology RH,”
Iron&Steel, 41(8) (2006), 1-11.
9. Y. Kato et al., “Development of Rapid Decarburization Technology by Combined Process of Converter and RH Degasser for Ultra Low Carbon Steel,” Zairyo to Purosesu (Current Advances in Materials and Processes), 3(1) (1990), 160-163.
10. J. Poirier, B. Thillou, M. Guiban, “Mechanisms and Counter Measures of Alumina Clogging in Submerged Nozzles” (Paper Presented at the 78th Steelmaking Conference Proceedings, Nashville Meeting, April 2-5 1995), 451-456.
11. D.Y. Wang et al., “Effect of Al Ash on Desulphurization of Pipe-line Steel,” China Metallurgy, 17(2) (2007), 14-16.
12. S.H. Wang et al., “Research on Physical Properties and Desulfurization Ability of 12CaO·7Al2O3 Refining Slag,” Journal of Hebei Institute of Technology, 21(8) (2001), 9-12.
13. G.H. LI, R. Zhu, T.C. Lin, “Study on High Efficient Desulfurization of New Type Synthetic Refining Slag,” Journal of University of Science and Technology Beijing, 31(12) (2009), 76-79.
14. G. Yuasa et al., “Refining Practice and Application of the Ladle Furnace (LF) Process in Japan,” Transactions of the Iron and Steel Institute of Japan, 24(5) (1984): 412-418.
15. Kishida T, Yajima T, Ukai A. “Recent Developments of Ladle Furnace (LF) Process in Japan” (Report on the Seventh International Conference on Vacuum Metallurgy: Special Meltings and Metallurgical Coatings, Keidanren Kaikan, Japan, November 26-30 1982), 1148- 1162.
